Tube pump and liquid ejection apparatus

ABSTRACT

A tube pump including a housing, a tube, a rotor, and a pressing member is disclosed. The tube has a section that is arranged in the housing in such a manner as to annularly extend along an inner circumferential surface. The rotor has a cam surface. The pressing member has a main body that selectively presses the tube toward the inner circumferential surface and a shaft that extends from the main body and contacts the cam surface. During rotation of the rotor, the pressing member moves, with the shaft contacting the cam surface, along the inner circumferential surface while pressing the tube. The shaft has a contact part that contacts the cam surface, and is formed in such a manner that the frictional coefficient of the shaft is lower than the frictional coefficient of the main body at least in the contact part.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2006-236909, filed on Aug. 31, 2006 and No. 2006-236911, filed on Aug. 31, 2006, the entire content of which is incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to a liquid ejection apparatus such as an inkjet printer and a tube pump used in a liquid ejection apparatus.

2. Related Art

Inkjet printers (hereinafter referred to as printers) are widely known as liquid injection apparatuses for injecting liquid from a nozzle formed in a recording head onto target. In a printer, since nozzles have openings on a nozzle-forming surface of a recording head (liquid ejection head), ink solvent evaporates from the nozzles. As a result, the viscosity of the ink increases, which may influence the ejection characteristics of the ink. Printers are therefore provided with a maintenance mechanism that performs maintenance of a recording head.

A maintenance mechanism includes a cap for sealing a nozzle-forming surface of the recording head and a suction pump located in a drain passage. The maintenance mechanism drives the suction pump while sealing the nozzle-forming surface with the cap, thereby producing negative pressure in the cap. Ink having an increased viscosity is drawn out of the nozzles, and the ink ejection is prevented from deteriorating. As such suction pumps, tub pumps that are disclosed, for example, in Japanese Laid-Open Patent Publication No. 2001-301195 and Japanese Laid-Open Patent Publication No. 2002-349452 are used.

The above publications each disclose a tube pump having a substantially cylindrical housing. The housing accommodates an intermediate portion of a flexible tube in the longitudinal direction, a rotor that rotates about the axis of the housing, and a roller (pressing member). When the rotor rotates in one direction, the roller moves along the inner circumferential surface of the housing while pressing the tube.

The tube is accommodated in the housing in such a manner that the intermediate portion in the longitudinal direction wound one turn along the inner circumferential surface of the housing to form a circle. Thus, in the housing, different sections of the tube partially overlap. An elongated hole that extends along the circumferential direction of the rotor is formed in a side of the rotor. The inner wall of the elongated hole defines a cam surface. The elongated hole extends in such a manner that the radial distance from the axis of the rotor is gradually reduced from one end in the circumferential direction to the other end. The roller has a shaft that extends along the axis, which shaft is slidably inserted in the elongated hole of the rotor. The roller is movable relative to the rotor between one end of the elongated hole (actuation position) and the other end (non-actuation position). As the roller moves from the non-actuation position toward the actuation position, the roller moves radially outward in the rotor.

When the rotor rotates in an actuation direction, which is one of the circumferential directions, the shaft of the roller slides toward the actuation position (pressing position) in the elongated hole. At the actuation position, the roller operates with the inner circumferential surface of the housing. That is, the roller moves along the inner circumferential surface of the housing, while consecutively pressing part of the intermediate portion of the tube. As the roller moves, portion of the tube that have been released from the pressing of the roller sequentially restore. Accordingly, the interior of a portion of the tube that is upstream of the intermediate portion, which is accommodated in the housing, is decompressed, so that negative pressure is produced in the cap. This draws ink with an increased viscosity from the nozzles.

When the tube pump is operating, the roller shaft moves along the elongated hole, and rotates at the actuation position. In these cases, frictional resistance is generated between the roller shaft and the cam surface. Particularly, when tube pump is performing suction, the roller shaft, at the actuation position, rotates and slides on the cam surface while receiving a reactive force from the pressed tube. Therefore, the frictional resistance is particularly increased when the roller is at the actuation position. Such frictional resistance is preferably minimized from the perspective of reducing the friction of the cam surface, reducing the torque required for operating the tube pump, and improving the pump efficiency.

However, the tube pumps disclosed in the above publications take no countermeasures to reduce the frictional resistance between a roller shaft and a cam surface.

SUMMARY

Accordingly, it is an objective of the present invention to provide a tube pump and a liquid ejection apparatus that reduce the torque required for operation and improves the pump efficiency.

To achieve the foregoing objective and in accordance with a first aspect of the present invention a tube pump having a housing, a tube, a rotor, and a pressing member is provided. The housing has an inner circumferential surface. The tube has a section that is arranged in the housing in such a manner as to annularly extend along the inner circumferential surface. The rotor is rotatably arranged in the housing, and has a cam surface. The pressing member is arranged in the housing. The pressing member has a main body that selectively presses the tube toward the inner circumferential surface and a shaft that extends from the main body and contacts the cam surface. During rotation of the rotor, the pressing member moves, with the shaft contacting the cam surface, along the inner circumferential surface while pressing the tube. The shaft has a contact part that contacts the cam surface. The shaft is formed in such a manner that the frictional coefficient of the shaft is lower than the frictional coefficient of the main body at least in the contact part.

In accordance with a second aspect of the present invention, a liquid ejection apparatus having a liquid ejection head, a cap, and a suction device is provided. The liquid ejection head has a nozzle-forming surface in which a nozzle for ejecting liquid is formed. The cap is capable of sealing the nozzle-forming surface. The suction device is capable of applying suction to the interior of the cap. The suction device is formed by the tube pump according to the first aspect of the present invention.

In accordance with a third aspect of the present invention, a tube pump having a housing, a tube, a rotor, and a pressing member is provided. The housing has an inner circumferential surface. The tube has a section that is arranged in the housing in such a manner as to annularly extend along the inner circumferential surface. The rotor is rotatably arranged in the housing, and has a cam surface. The pressing member is arranged in the housing. The pressing member has a main body that selectively presses the tube toward the inner circumferential surface and a shaft that extends from the main body and contacts the cam surface. During rotation of the rotor, the pressing member moves, with the shaft contacting the cam surface, along the inner circumferential surface while pressing the tube. The main body has an insertion hole into which the shaft is inserted. The shaft contacts the inner surface of the insertion hole and is rotatable relative to the main body. At a contacting portions of the shaft and the main body, the frictional coefficient of one of the shaft is lower than the frictional coefficient of the other.

In accordance with a fourth aspect of the present invention, a liquid ejection apparatus having a liquid ejection head, a cap, and a suction device is provided. The liquid ejection head has a nozzle-forming surface in which a nozzle for ejecting liquid is formed. The cap is capable of sealing the nozzle-forming surface. The suction device is capable of applying suction to the interior of the cap. The suction device is formed by the tube pump according to the third aspect of the present invention.

In accordance with a fifth aspect of the present invention, a tube pump having a housing, a tube, a rotor, and a pressing member is provided. The housing has an inner circumferential surface. The tube has a section that is arranged in the housing in such a manner as to annularly extend along the inner circumferential surface. The rotor is rotatably arranged in the housing. The rotor has a cam surface that extends in a circumferential direction of the rotor. The pressing member is arranged in the housing. The pressing member selectively presses the tube toward the inner circumferential surface and is slidable on the cam surface between an actuation position and a non-actuation position. During rotation of the rotor, the pressing member, if at the actuation position, moves along the inner circumferential surface while pressing the tube, thereby producing negative pressure in the tube, and, if at the non-actuation position, moves along the inner circumferential surface while pressing the tube in such a manner that no negative pressure is produced. The frictional coefficient of the cam surface is partially different.

In accordance with a sixth aspect of the present invention, a liquid ejection apparatus having a liquid ejection head, a cap, and a suction device is provided. The liquid ejection head has a nozzle-forming surface in which a nozzle for ejecting liquid is formed. The cap is capable of sealing the nozzle-forming surface. The suction device is capable of applying suction to the interior of the cap. The suction device is formed by the tube pump according to the fifth aspect of the present invention

Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:

FIG. 1 is a perspective view illustrating an inkjet printer according to one embodiment of the present invention;

FIG. 2 is a schematic cross-sectional view showing a part of the printer of FIG. 1;

FIG. 3 is a perspective view illustrating the tube pump of the printer shown in FIG. 1;

FIG. 4 is an exploded perspective view of the tube pump of FIG. 3;

FIG. 5 is a cross-sectional view of the tube pump of FIG. 3;

FIG. 6 is an exploded perspective view illustrating the pump wheel and the pressing member in the tube pump shown in FIG. 3; and

FIG. 7 is a side view showing the pressing member of FIG. 6.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

One embodiment of the present invention will now be described with reference to the drawings. Unless otherwise specified, a front-and-back direction, an up-and-down direction, and a left-and-right direction each agree with an arrow in all the drawings.

As shown in FIG. 1, an inkjet printer 11, which functions as a liquid ejection apparatus, has a frame 12, which is shaped like a rectangular box. A platen 13 is located in a lower portion of the frame 12. The platen 13 extends along the longitudinal direction, or left-and-right direction, of the frame 12. A paper feeding motor 14 is located in a lower back portion of the frame 12. Based on the driving force of the paper feeding motor 14, a paper feeding mechanism (not shown) feeds sheets of recording paper P forward from the rear side onto the platen 13.

A guide shaft 15 extends in the frame 12. The guide shaft 15 is located above the platen 13 and extends along the longitudinal direction of the platen 13. A carriage 16 is supported by the guide shaft 15, so that the carriage 16 reciprocates along the axial direction of the guide shaft 15. That is, the guide shaft 15 is passed through a support hole 16 a that extends through the carriage 16 in the left-and-right direction. The guide shaft 15 supports the carriage 16 such that the carriage 16 can reciprocate along the longitudinal direction of the guide shaft 15.

A drive pulley 17 a and a driven pulley 17 b are rotatably supported on a back wall of the frame 12 at positions corresponding to opposite ends of the guide shaft 15. An output shaft of a carriage motor 18 is connected to the drive pulley 17 a. An endless timing belt 17, which is coupled to the carriage 16, is wound around the pulleys 17 a, 17 b. The carriage 16 is thus moved in the left-and-right direction through the timing belt 17 while driven by the carriage motor 18 and guided by the guide shaft 15.

A recording head 19, or a liquid ejection head, is formed on a bottom surface of the carriage 16. Ink cartridges 20 are detachably mounted on the carriage 16. The ink cartridges 20 supply liquid, which is ink, to the recording head 19. The recording head 19 has piezoelectric elements 21 and nozzles 22 (see FIG. 2). When the piezoelectric elements 21 are activated, ink that has been supplied to the recording head 19 is injected onto the paper sheet P on the platen 13. Printing is thus performed.

A home position area (non-printing area), which does not correspond to the paper sheet P, is provided near the right end in the frame 12. A maintenance mechanism 23 is located in the home position area. The maintenance mechanism 23 performs maintenance such as cleaning of the recording head 19 when printing is not being performed.

Next, the maintenance mechanism 23 will be described.

As shown in FIG. 2, the maintenance mechanism 23 has cap 24 and a lift device 25. The cap 24 is shaped like a rectangular box with a bottom. The lift device 25 lifts and lowers the cap 24. The maintenance mechanism 23 moves the carriage 16 to the home position area, and in this state, lifts the cap 24 with the lift device 25, thereby sealing a nozzle-forming surface 19 a (the nozzles 22) of the recording head 19 with the cap 24. A projection 26 extends downward from the bottom of the cap 24. A drain passage 26 a for draining ink extends through the projection 26 along the up-and-down direction.

A first end (a proximal end or an upstream end) of a drain tube 27 made of a flexible material is connected to the projection 26. A second end (a distal end or a downstream end) of the drain tube 27 is inserted in a waste ink tank 28, which shaped like a rectangular parallelepiped. A tube pump 29, serving as a suction device, is located in an intermediate portion of the drain tube 27 between the cap 24 and the waste ink tank 28. The tube pump 29 draws air in the cap 24 from the cap 24 to the waste ink tank 28.

With the nozzle-forming surface 19 a being sealed with the cap 24, the tube pump 29 is activated so that viscous ink is drawn out of the nozzles 22 together with bubbles. The ink and bubbles are drained to the waste ink tank 28 through the cap 24 and the drain tube 27. This process is referred to as cleaning. A waste ink absorber 30 is accommodated in the waste ink tank 28. The waste ink absorber 30 absorbs and retains ink drained into the waste ink tank 28.

The tube pump 29 will now be described.

As shown in FIGS. 3 and 4, the tube pump 29 has a cylindrical housing 31 that is fixed in the frame 12 (see FIG. 1). The housing 31 has a bottom wall, and a through hole 31 a is formed in the bottom wall to connect the inside and the outside of the housing 31. A pump wheel 32 serving as a rotor is accommodated in the housing 31. The pump wheel 32 is rotatable about an axis of the housing 31. That is, the pump wheel 32 has a wheel shaft 33 that extends along the axis A and is inserted in the through hole 31 a. The pump wheel 32 is rotatable about the wheel shaft 33 in the housing 31.

An upstream opening (a first opening) 34 and a downstream opening (a second opening) 35 are formed in the circumferential wall. When viewed from above, the upstream opening 34 and the downstream opening 35 extend along a tangential direction of an inner circumferential surface 31 b of the housing 31. The upstream opening 34 and the downstream opening 35 are displaced from each other in the direction of the axis A. An intermediate portion 36 of the drain tube 27 in the longitudinal direction is accommodated in the housing 31, so as to be routed to draw a circle along the inner circumferential surface 31 b of the housing 31. The drain tube 27 extends to the outside of the housing 31 through the upstream opening 34 and the downstream opening 35.

In the housing 31, an upstream section 36 a and a downstream section 36 b in the intermediate portion 36 are partially overlap in the direction of the axis A. A section in which the upstream section 36 a and the downstream section 36 b partially overlap is defined as a tube overlapping section B. In the present embodiment, the drain tube 27 is wound one turn in the housing 31 such that the tube overlapping section B is minimized.

In the tube overlapping section B, the upstream section 36 a and the downstream section 36 b each form a thin portion 37 that is thinner than the remainder of the intermediate portion 36. The thickness of each thin portion 37 is determined such that the flexibility of the tube overlapping section B is close to the flexibility of the remainder of the intermediate portion 36 in the housing 31. That is, the thickness of each thin portion 37 is determined such that the flexibility of the drain tube 27 is constant over the entire circumferential direction in the housing 31.

As shown in FIG. 5, runout portions are provided on the inner circumferential surface 31 b of the housing 31 at positions corresponding to the upstream section 36 a and the downstream section 36 b. The runout portions can accommodate the upstream section 36 a and the downstream section 36 b. Specifically, an upstream recess 38 and a downstream recess 39 are formed in the inner circumferential surface 31 b of the housing 31. The upstream recess 38 serves as an upstream runout portion that corresponds to the upstream section 36 a. The downstream recess 39 serves as a downstream runout portion that corresponds to the downstream section 36 b. When viewed from above, the upstream recess 38 and the downstream recess 39 are arranged to be adjacent to each other in the circumferential direction of the inner circumferential surface 31 b of the housing 31.

As shown in FIG. 6, the pump wheel 32 has a disk-shaped large plate 40 and a small plate 41, which has a smaller diameter than that of the large plate 40. The large plate 40 and the small plate 41 are fixed to the wheel shaft 33 with the centers penetrated by the wheel shaft 33. The large plate 40 and the small plate 41 are spaced from each other at a predetermined distance along the axial direction. A roller guiding slit 42 extends through the large plate 40. The roller guiding slit 42 is arcuate and bulges radially outward. The roller guiding slit 42 has a first end and a second end. The roller guiding slit 42 extends in such a manner that the first end is located outside of the second end with respect to the radial direction of the large plate 40. That is, the roller guiding slit 42 extends so as to gradually approach the axis of the large plate 40 from the first end to the second end.

A roller guiding recess 43 is formed in a peripheral portion of the small plate 41. The roller guiding recess 43 corresponds to the roller guiding slit 42 of the large plate 40. Among the wall surfaces of the large plate 40 that define the roller guiding slit 42, the inner surface functions as a cam surface C. Also, a wall surface of the small plate 41 that defines the roller guiding recess 43 functions as a cam surface C.

A pressing member 46 is supported by the large plate 40 and the small plate 41 at the axial ends. The pressing member 46 includes a main body, which is a roller 44, and a shaft 45. A first end of the shaft 45 is inserted through the roller guiding slit 42 to be slidable on the cam surface C. A second end of the shaft 45 is arranged in the roller guiding recess 43 to be slidable on the cam surface C. That is, the roller 44 moves along the cam surfaces C with the first and second ends of the shaft 45 sliding on the cam surfaces C.

As shown in FIG. 7, the roller 44 and the shaft 45 are separate components. That is, the roller 44 has an insertion hole 44 a extending along the axis. The shaft 45, which is longer than the roller 44 in the axial direction is fitted in the insertion hole 44 a. Thus, the ends of the shaft 45 project from both ends of the roller 44. The shaft 45 is slidable on the inner surface of the insertion hole 44 a, so that the roller 44 and the shaft 45 are rotatable relative to each other.

A material having a lower frictional coefficient than that of the roller 44 is used for forming the shaft 45. Specifically, a metal or a synthetic resin having a low friction is used. A synthetic resin having a low friction includes resins of a sliding grade, such as polyacetal (POM) and polystyrene (PS). In the present embodiment, the material for forming the shaft 45, polyacetal of a sliding grade is used. Accordingly, the frictional coefficient of the shaft 45 is lower than that of the roller 44.

Retaining members, or retaining pins 47, for preventing the shaft 45 from coming off are provided at both ends of the roller 44. The retaining pins 47 prevent the shaft 45 from coming off the insertion hole 44 a of the roller 44. The retaining pins 47 are configured no to prevent relative rotation between the roller 44 and the shaft 45.

When a pump motor (not shown) is activated and the pump wheel 32 is rotated in an actuation direction of the pump 29, that is, in a direction causing tube the pump 29 to perform suction (a direction indicated by an arrow in FIG. 5), the pressing member 46 is moved to the first end of the roller guiding slit 42. That is, the pressing member 46 is located at a pressing position, which is a radially outer position in the pump wheel 32, or an actuation position. At the actuation position, the pressing member 46 moves from a upstream side to a downstream side along the longitudinal direction of the drain tube 27 as the roller 44 rotates, while pressing and squeezing the intermediate portion 36 of the drain tube 27.

As the pressing member 46 moves, a section of the drain tube 27 that is upstream of tube the pump 29 is decompressed. Air and ink in the cap 24, which seals the nozzle-forming surface 19 a, are gradually drained to the waste ink tank 28 by the rotation of the pump wheel 32 in the actuation direction, which produces negative pressure in the cap 24.

In contrast, when the pump wheel 32 is rotated in a direction opposite to the actuation direction (a direction opposite to the direction of the arrow in FIG. 5), the pressing member 46 is moved to the second end of the roller guiding slit 42. That is, the pressing member 46 is located at a non-pressing position, which is a radially inner position in the pump wheel 32, or a non-actuation position. At the non-actuation position, the pressing member 46 contacts the intermediate portion 36 of the drain tube 27 with a pressing force that is smaller compared to that in the case where the pressing member 46 is at the actuation position, and does not squeeze the drain tube 27. The decompressed state in the drain tube 27 is eliminated.

As shown in FIG. 6, when the pressing member 46 is moved to the first end of the roller guiding slit 42 on the cam surface of the pump wheel 32, the shaft 45 of the pressing member 46 contacts contact parts N. The contact parts N are formed of a material having a lower frictional coefficient than the remainder of the pump wheel 32. In the present embodiment, polytetrafluoroethylene (PTFE) is used as the low friction material.

The operation of the tube pump 29 will now be described.

When performing the cleaning of the recording head 19, the nozzles 22 (the nozzle-forming surface 19 a) of the recording head 19 are sealed by the cap 24. In this state, the pump motor (not shown) is activated and the pump wheel 32 is rotated in the actuation direction. Accordingly, the roller 44 is moved while rotating about the shaft 45, while squeezing the intermediate portion 36 of the drain tube 27 from the upstream side to the downstream side. At this time, both ends of the shaft 45 slide and rotate on the contact parts N on the cam surfaces C of the pump wheel 32, while the intermediate portion of the shaft 45 slides and rotate on the inner surface of the insertion hole 44 a.

In the present embodiment, the contact parts N are formed of polytetrafluoroethylene, which is a low friction material, and the shaft 45 is formed of polyacetal of a sliding grade. Therefore, the frictional resistance between the shaft 45 and the cam surfaces C and the frictional resistance between the shaft 45 and the roller 44 are both reduced.

When the roller 44 presses the tube overlapping section B, the upstream section 36 a and the downstream section 36 b of the drain tube 27 in the tube overlapping section B are moved to the upstream recess 38 and the downstream recess 39, respectively, so as to escape the pressing by the roller 44 at different timing. In addition, the thin portion 37 of the tube overlapping section B causes the flexibility of the tube overlapping section B to be close to the flexibility of the intermediate portion 36 except for the tube overlapping section B. Therefore, the fluctuation of torque when the intermediate portion 36 of the drain tube 27 is pressed by the roller 44 is suppressed.

When the roller 44 is moved on the intermediate portion 36 of the drain tube 27, while squeezing the upstream side to the downstream side of the intermediate portion 36, the interior of the drain tube 27 that is upstream of the tube pump 29 is decompressed, so that a negative pressure is produced in the cap 24. On the basis of the negative pressure, air and ink in the nozzles 22 and the cap 24 are drawn and drained to the waste ink tank 28 through the drain tube 27. Accordingly, the cleaning of the recording head 19 is completed.

The present embodiment provides the following advantages.

(1) In the pressing member 46, the shaft 45 is made of polyacetal of a sliding grade having lower frictional coefficient than that of the roller 44. Therefore, the frictional resistance between the shaft 45 and the cam surfaces C is lowered. This reduces the force required for rotating the pump wheel 32 when actuating the pump. Thus, the pump torque is reduced. Accordingly, the size of the pump motor (not shown) for driving the pump wheel 32 can be reduced, and the pump efficiency of the tube the pump 29 is improved. As a result, in the cleaning of the recording head 19, the interior of the cap 24 is efficiently and reliably vacuumed by the tube the pump 29.

(2) Since the shaft 45 and the roller 44 of the pressing member 46 are components formed separately from each other, the materials for the shaft 45 and the roller 44 are easily altered in accordance with the configuration of the tube the pump 29.

(3) The pressing member 46 is structured such that the shaft 45 and the roller 44 are rotatable relative to each other. The frictional resistance between the shaft 45 and the roller 44 is lower than the frictional resistance between the shaft 45 and the cam surfaces C, so that, during the activation of the pump, the roller 44 is mainly rotated relative to the shaft 45. Since this suppresses the friction between the shaft 45 and the cam surfaces C, the wear of the shaft 45 and the cam surfaces C is suppressed.

(4) The retaining pins 47 for preventing the shaft 45 from coming off the insertion hole 44 a of the roller 44 are attached to the shaft 45 of the pressing member 46. Therefore, the positional relation between the shaft 45 and the roller 44 is maintained.

(5) On the cam surfaces C of the pump wheel 32, the contact sections N are formed of polytetrafluoroethylene, which has a lower frictional coefficient than that of the remainder of the cam surfaces C. Thus, when the pressing member 46 presses the intermediate portion 36 of the drain tube 27, the sliding resistance between the shaft 45 and the cam surface C is reduced. Since this reduces the load during the actuation of tube the pump 29, the pump torque is reduced and the pump efficiency is improved.

(6) The pressing of the tube overlapping section B by the pressing member 46 corresponds to simultaneously pressing of two sections of the drain tubes 27 by the pressing member 46. In this respect, since the present embodiment provides the thin portion 37 in the tube overlapping section B, the tube overlapping section B is easily flexed. Therefore, the load required for squeezing the tube overlapping section B with the pressing member 46 is reduced by a simple structure that only has the thin portion 37. Thus, the pump torque is reduced and the pump efficiency is improved.

(7) The flexibility of the tube overlapping section B is made close to the flexibility of portions other than the tube overlapping section B. That is, the load required for squeezing the tube overlapping section B with the pressing member 46 is made close to the load required for squeezing sections of the intermediate portion 36 other than the tube overlapping section B. Thus, the load required for squeezing the intermediate portion 36 is equalized over the entire length of the intermediate portion 36 in the housing 31. Therefore, fluctuation of the pump torque is suppressed. This allows the tube the pump 29 to operate in a stable manner.

(8) The upstream recess 38 and the downstream recess 39 are formed in the housing to correspond to the upstream section 36 a and the downstream section 36 b, which form the tube overlapping section B, respectively. When the tube overlapping section B is pressed by the pressing member 46, the upstream section 36 a and the downstream section 36 b moved to the corresponding recesses 38, 39, respectively, so as to escape the pressing. Therefore, the load required for squeezing the tube overlapping section B with the pressing member 46 is effectively reduced. This reduces the pump torque and improves the pump efficiency.

(9) Since the upstream recess 38 and the downstream recess 39 are arranged in the housing 31 to be adjacent to each other along the circumferential direction of the inner circumferential surface 31 b of the housing 31, the intermediate portion 36 of the drain tube 27 can be routed in the housing 31 to minimize the tube overlapping section B. Therefore, when the pump is used, the load for squeezing the intermediate portion 36 of the drain tube 27 with the pressing member 46 is made equal to a load required for squeezing a single drain tube 27 along the entire circumference of the housing 31, while substantially eliminating the amount of leak. Thus, the pump torque is reduced and the pump efficiency is improved.

The present invention is not limited to the above described embodiment, but may be embodied as follows, for example.

The upstream section 36 a and the downstream section 36 b of the tube overlapping section B may be made of a material having a lower rigidity than that of sections of the intermediate portion 36 other than the tube overlapping section B, thereby increasing the flexibility of the tube overlapping section B. This reliably reduces the load required for squeezing the tube overlapping section B with the pressing member 46.

The frictional coefficient on the cam surfaces C of the pump wheel 32 may be partially changed in accordance with the configuration of the tube the pump 29. This allows the pressing member 46 to press the drain tube 27 in a favorable manner, and to smoothly move between the pressing position and the non-pressing position. As a result, the pump torque is reduced and the pump efficiency is improved.

The contact parts N in the cam surfaces C may be formed as components separate from the large plate 40, and made of a low friction material such as polytetrafluoroethylene and polyacetal or polystyrene of a sliding grade. In this case, the low frictional resistance members are molded products that are press fitted to the large plate 40. Accordingly, the sliding resistance between the shaft 45 and the contact parts N when the pressing member 46 presses the intermediate portion 36 of the drain tube 27 is reliably reduced by a simple structure.

Low friction coating such as polytetrafluoroethylene or grease for reducing frictional resistance may be applied to the contact parts N of the cam surfaces C. Accordingly, the sliding resistance between the shaft 45 and the contact parts N when the pressing member 46 presses the intermediate portion 36 of the drain tube 27 is easily and reliably reduced.

The contact parts N on the cam surfaces C may be subject to surface treatment such as polishing, thereby reducing the frictional coefficient of the contact parts N.

The retaining pins 47 may be replaced by seal rings functioning as retaining members.

Without reducing the frictional resistance between the shaft 45 of the pressing member 46 and the cam surfaces C of the pump wheel 32, the frictional coefficient of one of the sliding section of the shaft 45 and the sliding section of the roller 44 may be set lower than the frictional coefficient of the other. Since this reduces the frictional resistance of the shaft 45 and the roller 44, only the roller 44 is rotated without causing the shaft 45 to rotate when the pump wheel 32 is rotated during the activation of the pump. Therefore, the load applied to the cam surfaces C from the shaft 45 is reduced, which reduces the wear of the cam surfaces C caused by the shaft 45. Further, the frictional resistance between the shaft 45 and the roller 44. This reduces the force required for rotating the pump wheel 32 when actuating the pump. Thus, the pump torque is reduced and the pump efficiency is improved.

The shaft 45 may be press fitted to the insertion hole 44 a of the roller 44 so that the roller 44 and the shaft 45 rotate integrally.

The shaft 45 of the pressing member 46 may be made of a low friction material such as polytetrafluoroethylene.

Only parts of the shaft 45 of the pressing member 46 that slide on the cam surfaces C, that is, only both end portions of the shaft 45 may be formed of a low friction material such as polytetrafluoroethylene, metal, and polyacetal or polystyrene of a sliding grade.

The roller 44 and the shaft 45 may be formed as an integral body.

The upstream recess 38 and the downstream recess 39 do not need to be arranged to be adjacent to each other in the circumferential direction of the inner circumferential surface 31 b of the housing 31.

Either one of the upstream recess 38 and the downstream recess 39 may be omitted.

Instead of the roller 44 and the shaft 45, the pressing member 46 may include a sliding member that slides on the intermediate portion 36 of the drain tube 27, while pressing the intermediate portion 36.

In the illustrated embodiment, the liquid ejection apparatus, which is equipped with the tube pump 29, is embodied as the inkjet printer 11. However, for example, the present invention may be embodied as a liquid ejection apparatus used for manufacturing color filters for liquid crystal displays or pixels of organic EL displays. Alternatively, the tube pump 29 may be mounted on apparatus other than liquid ejection apparatuses. 

1. A tube pump comprising: a housing having an inner circumferential surface; a tube having a section that is arranged in the housing in such a manner as to annularly extend along the inner circumferential surface; a rotor rotatably arranged in the housing, the rotor having a cam surface; and a pressing member arranged in the housing, the pressing member having a main body that selectively presses the tube toward the inner circumferential surface and a shaft that extends from the main body and contacts the cam surface, wherein, during rotation of the rotor, the pressing member moves, with the shaft contacting the cam surface, along the inner circumferential surface while pressing the tube, wherein the shaft has a contact part that contacts the cam surface, and the shaft is formed in such a manner that the frictional coefficient of the shaft is lower than the frictional coefficient of the main body at least in the contact part.
 2. The pump according to claim 1, wherein the shaft and the main body are components formed separately from each other, and the main body has an insertion hole into which the shaft is inserted.
 3. The pump according to claim 2, wherein the pressing member includes a retaining member for preventing the shaft from coming off the insertion hole.
 4. The pump according to claim 1, wherein the contact part is formed of a metal or a synthetic resin having a low friction.
 5. The pump according to claim 1, wherein the shaft is rotatable relative to the main body.
 6. A liquid ejection apparatus comprising: a liquid ejection head having a nozzle-forming surface in which a nozzle for ejecting liquid is formed; a cap capable of sealing the nozzle-forming surface; and a suction device capable of applying suction to the interior of the cap, wherein the suction device is formed by the tube pump according to claim
 1. 7. A tube pump comprising: a housing having an inner circumferential surface; a tube having a section that is arranged in the housing in such a manner as to annularly extend along the inner circumferential surface; a rotor rotatably arranged in the housing, the rotor having a cam surface; and a pressing member arranged in the housing, the pressing member having a main body that selectively presses the tube toward the inner circumferential surface and a shaft that extends from the main body and contacts the cam surface, wherein, during rotation of the rotor, the pressing member moves, with the shaft contacting the cam surface, along the inner circumferential surface while pressing the tube, wherein the main body has an insertion hole into which the shaft is inserted, the shaft contacting the inner surface of the insertion hole and being rotatable relative to the main body, and wherein, at a contacting portions of the shaft and the main body, the frictional coefficient of one of the shaft is lower than the frictional coefficient of the other.
 8. A liquid ejection apparatus comprising: a liquid ejection head having a nozzle-forming surface in which a nozzle for ejecting liquid is formed; a cap capable of sealing the nozzle-forming surface; and a suction device capable of applying suction to the interior of the cap, wherein the suction device is formed by the tube pump according to claim
 7. 9. A tube pump comprising: a housing having an inner circumferential surface; a tube having a section that is arranged in the housing in such a manner as to annularly extend along the inner circumferential surface; a rotor rotatably arranged in the housing, the rotor having a cam surface that extends in a circumferential direction of the rotor; and a pressing member arranged in the housing, the pressing member selectively pressing the tube toward the inner circumferential surface and being slidable on the cam surface between an actuation position and a non-actuation position, wherein, during rotation of the rotor, the pressing member, if at the actuation position, moves along the inner circumferential surface while pressing the tube, thereby producing negative pressure in the tube, and, if at the non-actuation position, moves along the inner circumferential surface while pressing the tube in such a manner that no negative pressure is produced, wherein the frictional coefficient of the cam surface is partially different.
 10. The pump according to claim 9, wherein the cam surface has a contact part, the pressing member contacting the contact part when at the actuation position, and wherein the frictional coefficient of the contact part is lower than the frictional coefficient of the remainder of the cam surface.
 11. The pump according to claim 10, wherein the contact part is formed of a material having a lower frictional coefficient than the friction coefficient of the remainder of the cam surface.
 12. The pump according to claim 10, wherein the contact part is formed as a component separate from the pressing member and is made of a material having a lower frictional coefficient than the friction coefficient of the remainder of the cam surface.
 13. The pump according to claim 10, wherein coating for reducing frictional resistance is applied to the contact part.
 14. A liquid ejection apparatus comprising: a liquid ejection head having a nozzle-forming surface in which a nozzle for ejecting liquid is formed; a cap capable of sealing the nozzle-forming surface; and a suction device capable of applying suction to the interior of the cap, wherein the suction device is formed by the tube pump according to claim
 9. 